Polyimide (PI) has a variety of uses in the semiconductor industry. It can be used in place of a conventional silicon dioxide (SiO.sub.2) or silicon nitride (Si.sub.3 N.sub.4) layer as an interlayer dielectric. In this case it serves as an insulative layer to prevent shorting between features or devices formed from a first conductive layer and features or devices formed from a subsequent conductive layer.
Polyimide can also be used as a passivation layer or protective overcoat. When used as a passivation layer its purpose is to protect the integrated circuit from scratches and other physical defects. That is, it is used as a protective coating to improve yield and reliability. For a discussion of the uses and advantages of polyimide see "Polyimide Coatings for Microelectronic Applications," Y. K. Lee and J. D. Craig, Polymer Materials for Electronic Applications, American Chemical Society, page 107 and "Polyimides for Use as VLSI Multilevel Interconnection Dielectric and Passivation Layer," Thomas E. Wade, Microscience, Polyimides for VLSI Dielectric and Passivation Layer, page 61.
In the semiconductor industry, polyimide is normally spun on the wafer. A pool of liquid polyimide is poured on the substrate and the substrate is spun at high speeds, typically about 3,000 rpm, to create a polyimide layer of uniform thickness across the surface of the substrate. The polyimide is next partially cured or imidized. In most applications the polyimide must then be patterned. Normally a photoresist layer is applied next and is spun on in a process similar to the spinning of the polyimide. Next the photoresist is exposed to light to define the regions which will be developed away. If a positive photoresist is used the regions exposed to light will dissolve in the developer leaving the desired pattern. If a negative photoresist is used the regions not exposed to light will be dissolved in developer.
After the pattern has been defined by the photoresist as described above the polyimide is etched. In the case of positive photoresist the developer used for the photoresist also etches polyimide, thus the developing of the photoresist and the etching of the polyimide are carried out in one step. When negative photoresist is used, sodium hydroxide, potassium hydroxide or tetra alkyl ammonium hydroxide can be used as an etchant for the polyimide. Additionally the polyimide can be dry etched in a plasma etch using an oxygen gas. If this method is used the photoresist thickness normally must be greater than that of the polyimide since in most processes they etch at the same rate as one another in a plasma etch.
After etching of the polyimide in the processes described above the photoresist must be removed or stripped. Generally the photoresist is approximately the same thickness as when spun on, that is, the photeresist developing and polyimide etching processes do not significantly reduce the thickness of the photoresist layer.
The stripping of the photoresist is normally carried out in a wet process with the solvents N-butyl acetate (NBA) and isopropanol (IPA). As an example of a wet process to strip photoresist, the wafers are sequentially dipped in two NBA baths at room temperature for 10 minutes in each. They are next dipped in an IPA bath at room room temperature for 10 minutes. After the NBA and IPA dips, the wafers may be rinsed in water and then spun dry.
There are several disadvantages to the wet photoresist stripping method described above. The process is not cost effective. Both NBA and IPA evaporate quickly at room temperature resulting in a high consumption rate of the chemicals. In addition the process is labor intensive with low throughput and many processing steps.
The wet stripping method also has safety and environmental problems. Because of the frequent handling required in the process, operators are frequently exposed to solvent vapor. In addition the high consumption rates of these chemicals is an environmental hazard.
The wet stripping method also suffers from poor process control. It can leave residual photoresist on the polyimide surface and requires a short plasma clean afterwards. In addition the polyimide layer itself is sometimes lifted up from the surface it has been applied to during the course of the photoresist stripping by the solvents used in the wet method.
Because of these disadvantages of the wet stripping method, it is preferred to strip the photoresist in a dry process whenever possible. Most commonly an oxygen plasma is used and the process is called plasma etching or ashing. In these processes an oxygen plasma comprising oxygen radicals is formed. The oxygen reacts with the photoresist to form water and various other compounds thereby stripping the photoresist layer. For example, when stripping photoresist off of a silicon dioxide (SiO.sub.2) surface, there is extremely good selectivity. That is, the oxygen plasma will etch photoresist without etching the SiO.sub.2. Therefore, a batch of substrates can be placed in an oxygen plasma for a timed period. The time period usually includes an over-etch to ensure complete removal of the photoresist. Since the selectivity in etching photoresist is good, there is no need to determine precisely when the underlying SiO.sub.2 surface has been reached and it is not detrimental to over-etch to the extent necessary to overcome non-uniformities in the photoresist thickness or non-uniformities in the etching process.
However, plasma etch has not been used for removing photoresist from polyimide due to the fact that both are similar organic materials and in most processes it is found that they etch at about the same rate. Additionally, it is thought that because of the similarities in their chemical make-ups there is no way to reliably detect an endpoint of the photoresist stripping. Because of these factors, it is possible to etch away much of the patterned polyimide layer in a timed dry photoresist stripping process.
What is needed therefore is a dry process for stripping photoresist from a polyimide surface with good photoresist to polyimide selectivity where the endpoint of the photoresist stripping can be reliably determined and the strip can be stopped after complete removal of the photoresist but before significant amounts of polyimide have been etched.